This invention relates generally to light emitting diodes, and more particularly to using light emitting diodes as light sensors as well as light emitters.
Remote controls, personal digital assistants (PDAs), pagers, cell phones and other handheld devices often include a backlight for display, keys, and control buttons so that the devices can be used in dark environments. Generally, the user must press a button, open a lid, or take some other sort of action to turn on the backlight. However, locating the backlight actuation mechanism can be difficult to do in the dark. Backlights are also extensively used in passive display panels that are subject to various levels of ambient light, for example, vehicle and plane instrument panels.
Some conventional mechanisms turn on the backlight by any user action. This provides multiple ways to activate the backlight, but the user must be careful not to hit an undesirable function key in the dark by mistake. Worse still, this mechanism turns on the backlight even in bright environments, when it is not needed. Because battery life is critical in handheld devices, some designers have added light sensors to prevent actuation in bright light conditions, but this adds complexity and cost.
Another prior art mechanism uses a combination of accelerometers, capacitive touch sensors, and infrared proximity sensors to determine a handling state of a mobile phone to turn on a backlight, see Hinckley et al., xe2x80x9cToward More Sensitive Mobile Phones,xe2x80x9d Proceedings of ACM UIST, 2001. Not only are these components expensive, they also draw a substantial amount of power and require significant tooling changes for the design of the device""s enclosure. A simpler mechanism uses a simple capacitive proximity sensor as described in U.S. patent application Ser. No. 09/864,812, xe2x80x9cReal-Time Buffering for Telephone Handsets,xe2x80x9d filed by Dietz et al. on May 24, 2001, but with a slight modification to detect motion rather than simply crossing a threshold.
Unlike conventional systems, the automatic backlight turns on before the user hits a key, but only in dark lighting conditions. It is inexpensive, and uses very little power. A capacitive proximity sensor detects active handling of the device, and the backlight itself is also used as a light sensor. The circuit requires only a single additional capacitor and three I/O pins of a typical microprocessor-based systems (such as remote controls). Because the system also uses the backlight LED as a sensor, devices can often be upgraded to include this feature without changing the existing packaging.